Municipal solid waste (MSW) includes all solid materials disposed by municipalities. While some of this waste is recycled, the majority is typically dumped in landfills, where it decomposes over a period of decades or even centuries. It has been recognized that municipal solid waste contains organic materials that have energy content. If MSW is left untreated in landfills, the energy content can be drained slowly from the landfill by bacterial processes, which not only dissipate the concentrated energy but, also, produce methane, a strong greenhouse gas. Some landfills have sought to collect methane, which may be used for fuel; however, the conversion to methane takes place on long time scales, wastes much of the internal energy of the MSW, and is rather ineffective in recovering much of the available energy content of the MSW.
The earliest and most common method of recovering energy from MSW is incineration. Incineration includes the combustion of MSW or refuse-derived fuel (RDF) to produce heat, which typically powers a turbine to produce electricity. Byproducts of incineration include fly ash, bottom ash, and flue gases containing dangerous pollutants including sulfur compounds, CO2, which is a green-house gas, acid gases as well as metals, metal compounds and particulates. Fly ash and bottom ash are typically discarded in landfills. Some harmful flue gases and particulates can be scrubbed from the incineration flue stream prior to discharge into the atmosphere.
Another method of recovering energy from MSW is pyrolysis, which involves heating the organic portions of the MSW, so that thermally unstable compounds are chemically decomposed into other compounds. Those compounds mix with other volatile components to form a pyrolysis gas that typically includes tars, alkenes, aromatic hydrocarbons, sulfur compounds, steam, and carbon dioxide. The solid residue from pyrolysis process includes coke (residual carbon), which can then be burned or used as a gasification feedstock.
A related method for recovering energy from MSW is gasification. Gasification involves converting at least a fraction of the MSW into a synthesis gas (“syngas’) composed mainly of carbon monoxide carbon dioxide, and hydrogen. Gasification technology has existed for some centuries. In the nineteenth century, for instance, coal and peat were often gasified into “town gas” that provided a flammable mix of carbon monoxide (CO), methane (CH4) and hydrogen (H2) that was used for cooking, heating and lighting. During World Wars I and II, biomass and coal gasifies were used to produce CO and H2 to meet transportation needs. Sometimes, some of the syngas was converted directly in to liquid transportation fuels using the Fisher-Tropsch process. With the discovery of vast quantities of domestic oil and natural gas following World War II, coal and biomass gasification were no longer cost-competitive and all but disappeared.
Gasification has been applied directly to the MSW but, in other cases, the MSW is first pyrolyzed, and then subjected to a secondary gasification process. Gasification of MSW generally includes a mechanical processing step that removes recyclables and other materials that have low or no energy content. Then, the processed feedstock is heated in a gasifier in the presence of a gasification agent (including at least some oxygen and possibly steam). Gasifiers may have a number of configurations. For example, fixed-bed gasifiers place the feedstock in a fixed bed, and then contact it with a stream of a gasification agent in either a counter-current (“up draft”) or co-current (“down draft”) manner. Gasifiers may also use fluidized bed reactors.
Another method of gasifying MSW is treatment in the presence of oxygen with a high-temperature plasma. Such systems may convert the MSW to syngas, leaving vitrified wastes and metals as byproduct.
To create hydrocarbons as synthetic fuels, a known method for converting syngas into synthetic fuels is the catalytic Fischer-Tropsch (F-T) process. This process produces a mixture of hydrocarbons which could be further refined to produce liquid transportation fuels.
With numerous detrimental effects of greenhouse gases being increasingly documented, there is a clear need to reduce energy production from fossil fuels, particularly from petroleum and coal-derived fuel sources. To encourage the reduction of fossil fuel usage, governments are promoting the usage of fuels derived from renewable organic sources rather than fossil-based sources.
The Environmental Protection Agency (EPA) in the United States has mandated a Renewable Fuel Standard (“RFS”) under which cellulosic-based fuels generate Cellulosic RINs (renewable identification numbers) which are a form of compliance credits for Obligated Parties (e.g., refineries). Under the RFS, the Obligated Parties are required to blend an increasing amount of cellulosic fuel into fossil-derived fuels.
To determine the biogenic percentage content of fuels, the EPA requires tests that use radiocarbon dating methods. More particularly, current the USEPA regulations, at Section 8.1426(f)(9), require parties to use Method B or Method C of ASTM D 6866 to perform radiocarbon dating to determine the renewable fraction of the fuel.